Wide band gap semiconductor materials such as silicon carbide have a higher tolerance to breakdown voltage than that of silicon, allowing for an increase in impurity concentration of a substrate and a decrease in resistance of the substrate in comparison with the silicon material. The decrease in resistance of the substrate can reduce a loss in switching operations of a power element. The wide band gap semiconductor materials in comparison with silicon have a higher thermal conductivity and also have excellent mechanical strength. Thus, the wide band gap semiconductor materials are expected to be materials capable of achieving small, low-loss, high-efficiency power semiconductor devices.
Power semiconductor devices including silicon carbide (silicon carbide power semiconductor devices) typically have a current path in a direction perpendicular to a substrate face for securing a density of current. Further, the power semiconductor devices including silicon carbide (silicon carbide power semiconductor devices) include a drift layer that is located on a silicon carbide substrate and that has a doping concentration lower than that in the substrate for securing breakdown voltage of the device.
The silicon carbide power semiconductor device has high breakdown voltage in recent times, and thus the drift layer has a greater thickness, causing a longer current path in the drift layer. Consequently, drift resistance forms an increasing proportion of ON resistance, which increases a conduction loss of the device.
The greater thickness of the drift layer increases distortion of lattice in the drift layer due to a mismatch in lattice constant between the drift layer and the substrate, and thus a density of crystal defects in the drift layer increases, causing an increase in resistance and a decrease in yields of the device. For this reason, it is an extremely important challenge to achieve the structure of the device capable of reducing a drift resistance and a density of crystal defects.
Although a drift layer typically has a fixed doping concentration in a film thickness direction, Patent Document 1, for example, discloses a doping concentration distribution of a drift layer having an impurity concentration continuously decreased in a growth direction, which can reduce the drift resistance more than the case where the doping concentration is fixed.
Patent Document 2, for example, discloses that a density of crystal defects can be reduced by forming a buffer layer, which gradually reduces an impurity concentration, between a substrate and a drift layer while the drift layer has the fixed doping concentration in the film thickness direction.